Decisions and Loops

Introduction

So far every program ran straight down, line by line. Real programs decide ("is the password correct?") and repeat ("do this for each item"). These two ideas, conditions and loops, are the heart of programming.

Comparisons give True or False

The operators are ==, !=, <, >, <= and >=. Remember that a single = stores a value, while == asks whether two values are equal.

if, elif, else

Indentation matters in Python. The lines indented under if belong to it, and Python uses the indentation, usually four spaces, to know where the block ends.

Python checks the conditions from top to bottom and runs the first one that is true. Change temperature and run it several times.

and, or, not

Combine conditions with the words and, or and not.

You can chain comparisons the way people write maths: 12 <= age < 18 is true for ages 12 to 17.

Truthy and falsy

Python treats some values as false in an if: 0, an empty string "", an empty list [] and None. Everything else counts as true.

for loops

A for loop does something once for each item in a sequence.

range() makes a sequence of numbers. range(5) is 0, 1, 2, 3, 4, and it stops before the end number.

You can loop over the letters of a string too:

while loops

A while loop repeats as long as its condition is true. Make sure something inside the loop changes, or it will never stop.

break and continue

• break leaves the loop immediately.
• continue skips to the next round.

Putting it together: FizzBuzz

A classic exercise: for numbers 1 to 15, print Fizz if divisible by 3, Buzz if divisible by 5, FizzBuzz if both, otherwise the number.

Notice the order: we test for 15 first, because a number divisible by 15 is also divisible by 3 and 5.

Try it yourself

Write a loop that adds up the numbers from 1 to 100 and prints the total (the answer is 5050). Then change it to add up only the even numbers.

Summary

• Comparisons (==, !=, <, >) give True or False. Combine them with and, or, not.
• if, elif and else choose a path, and indentation marks the block.
• for loops over a sequence, and range(n) counts from 0 up to n - 1. while repeats while a condition holds.
• break leaves a loop and continue skips to the next round.